Yes, there's definitely reason to have trust in Whedon and the one thing that makes it easier for him to do this is that Hulk doesn't have to carry the movie on his own. In a new solo movie he'd have to do that while in TA2 the main dynamic will be that between the members of the team.

Good point. I guess he'll need more of a personal conflict in a solo film. Maybe losing a friend and then outright killing some bad guys.